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97-472: The goal was to test whether selected organisms can survive an undetermined number of years in deep space by flying them through interplanetary space. The experiment would have tested one aspect of panspermia , the hypothesis that life could survive space travel, if protected inside rocks blasted by impact off one planet to land on another. Prior to the Phobos LIFE experiment, a precursor LIFE prototype

194-466: A futile cycle . Although fat is a common way of storing energy, in vertebrates such as humans the fatty acids in these stores cannot be converted to glucose through gluconeogenesis as these organisms cannot convert acetyl-CoA into pyruvate ; plants do, but animals do not, have the necessary enzymatic machinery. As a result, after long-term starvation, vertebrates need to produce ketone bodies from fatty acids to replace glucose in tissues such as

291-470: A permafrost region on Earth – within a cavity 26 mm in diameter. The module passed stress tests including a shake test with vibrations at frequencies to 1,100 Hz and an impact test of 4,000 g, designed to simulate the potential impact of the capsule on Earth. The LIFE experiment was launched on November 8, 2011 on board the Fobos-Grunt . However, the spacecraft failed to depart Earth orbit due to

388-562: A siphonophore , a jelly-like marine animal, is composed of organism-like zooids , but the whole structure looks and functions much like an animal such as a jellyfish , the parts collaborating to provide the functions of the colonial organism. The evolutionary biologists David Queller and Joan Strassmann state that "organismality", the qualities or attributes that define an entity as an organism, has evolved socially as groups of simpler units (from cells upwards) came to cooperate without conflicts. They propose that cooperation should be used as

485-440: A DNA template from its viral RNA genome. RNA in ribozymes such as spliceosomes and ribosomes is similar to enzymes as it can catalyze chemical reactions. Individual nucleosides are made by attaching a nucleobase to a ribose sugar. These bases are heterocyclic rings containing nitrogen, classified as purines or pyrimidines . Nucleotides also act as coenzymes in metabolic-group-transfer reactions. Metabolism involves

582-423: A constant set of conditions within cells, a condition called homeostasis . Metabolic regulation also allows organisms to respond to signals and interact actively with their environments. Two closely linked concepts are important for understanding how metabolic pathways are controlled. Firstly, the regulation of an enzyme in a pathway is how its activity is increased and decreased in response to signals. Secondly,

679-429: A cycle of reactions that add the acyl group, reduce it to an alcohol, dehydrate it to an alkene group and then reduce it again to an alkane group. The enzymes of fatty acid biosynthesis are divided into two groups: in animals and fungi, all these fatty acid synthase reactions are carried out by a single multifunctional type I protein, while in plant plastids and bacteria separate type II enzymes perform each step in

776-569: A few enzymes and molecules like those in living organisms, they have no metabolism of their own; they cannot synthesize the organic compounds from which they are formed. In this sense, they are similar to inanimate matter. Viruses have their own genes , and they evolve . Thus, an argument that viruses should be classed as living organisms is their ability to undergo evolution and replicate through self-assembly. However, some scientists argue that viruses neither evolve nor self-reproduce. Instead, viruses are evolved by their host cells, meaning that there

873-398: A group could be viewed as a superorganism , optimized by group adaptation . Another view is that attributes like autonomy, genetic homogeneity and genetic uniqueness should be examined separately rather than demanding that an organism should have all of them; if so, there are multiple dimensions to biological individuality, resulting in several types of organism. A unicellular organism

970-458: A hydrogen acceptor. Hundreds of separate types of dehydrogenases remove electrons from their substrates and reduce NAD into NADH. This reduced form of the coenzyme is then a substrate for any of the reductases in the cell that need to transfer hydrogen atoms to their substrates. Nicotinamide adenine dinucleotide exists in two related forms in the cell, NADH and NADPH. The NAD /NADH form is more important in catabolic reactions, while NADP /NADPH

1067-558: A large group of compounds that contain fatty acids and glycerol ; a glycerol molecule attached to three fatty acids by ester linkages is called a triacylglyceride . Several variations of the basic structure exist, including backbones such as sphingosine in sphingomyelin , and hydrophilic groups such as phosphate in phospholipids . Steroids such as sterol are another major class of lipids. Carbohydrates are aldehydes or ketones , with many hydroxyl groups attached, that can exist as straight chains or rings. Carbohydrates are

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1164-404: A larger increase in the entropy of their environments. The metabolism of a cell achieves this by coupling the spontaneous processes of catabolism to the non-spontaneous processes of anabolism. In thermodynamic terms, metabolism maintains order by creating disorder. As the environments of most organisms are constantly changing, the reactions of metabolism must be finely regulated to maintain

1261-472: A membrane. Pumping protons out of the mitochondria creates a proton concentration difference across the membrane and generates an electrochemical gradient . This force drives protons back into the mitochondrion through the base of an enzyme called ATP synthase . The flow of protons makes the stalk subunit rotate, causing the active site of the synthase domain to change shape and phosphorylate adenosine diphosphate —turning it into ATP. Chemolithotrophy

1358-404: A process of recombination (a primitive form of sexual interaction ). Metabolism Metabolism ( / m ə ˈ t æ b ə l ɪ z ə m / , from Greek : μεταβολή metabolē , "change") is the set of life -sustaining chemical reactions in organisms . The three main functions of metabolism are: the conversion of the energy in food to energy available to run cellular processes;

1455-559: A programming error, and fell back to Earth in the Pacific Ocean. The module was not recovered. The team is seeking out future exploratory opportunities. Organism An organism is defined in a medical dictionary as any living thing that functions as an individual . Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have been proposed to define what an organism is. Among

1552-591: A set of xenobiotic-metabolizing enzymes. In humans, these include cytochrome P450 oxidases , UDP-glucuronosyltransferases , and glutathione S -transferases . This system of enzymes acts in three stages to firstly oxidize the xenobiotic (phase I) and then conjugate water-soluble groups onto the molecule (phase II). The modified water-soluble xenobiotic can then be pumped out of cells and in multicellular organisms may be further metabolized before being excreted (phase III). In ecology , these reactions are particularly important in microbial biodegradation of pollutants and

1649-459: A short ancestral pathway, the duplication and then divergence of entire pathways as well as the recruitment of pre-existing enzymes and their assembly into a novel reaction pathway. The relative importance of these mechanisms is unclear, but genomic studies have shown that enzymes in a pathway are likely to have a shared ancestry, suggesting that many pathways have evolved in a step-by-step fashion with novel functions created from pre-existing steps in

1746-530: A source of energy, while switching between carbon fixation and the fermentation of organic compounds. In many organisms, the capture of solar energy is similar in principle to oxidative phosphorylation, as it involves the storage of energy as a proton concentration gradient. This proton motive force then drives ATP synthesis. The electrons needed to drive this electron transport chain come from light-gathering proteins called photosynthetic reaction centres . Reaction centers are classified into two types depending on

1843-413: A vast array of chemical reactions, but most fall under a few basic types of reactions that involve the transfer of functional groups of atoms and their bonds within molecules. This common chemistry allows cells to use a small set of metabolic intermediates to carry chemical groups between different reactions. These group-transfer intermediates are called coenzymes . Each class of group-transfer reactions

1940-419: Is oxidative stress . Here, processes including oxidative phosphorylation and the formation of disulfide bonds during protein folding produce reactive oxygen species such as hydrogen peroxide . These damaging oxidants are removed by antioxidant metabolites such as glutathione and enzymes such as catalases and peroxidases . Living organisms must obey the laws of thermodynamics , which describe

2037-445: Is a microorganism such as a protist , bacterium , or archaean , composed of a single cell , which may contain functional structures called organelles . A multicellular organism such as an animal , plant , fungus , or alga is composed of many cells, often specialised. A colonial organism such as a siphonophore is a being which functions as an individual but is composed of communicating individuals. A superorganism

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2134-401: Is a teleonomic or goal-seeking behaviour that enables them to correct errors of many kinds so as to achieve whatever result they are designed for. Such behaviour is reminiscent of intelligent action by organisms; intelligence is seen as an embodied form of cognition . All organisms that exist today possess a self-replicating informational molecule (genome), and such an informational molecule

2231-741: Is a colony, such as of ants , consisting of many individuals working together as a single functional or social unit . A mutualism is a partnership of two or more species which each provide some of the needs of the other. A lichen consists of fungi and algae or cyanobacteria , with a bacterial microbiome ; together, they are able to flourish as a kind of organism, the components having different functions, in habitats such as dry rocks where neither could grow alone. The evolutionary biologists David Queller and Joan Strassmann state that "organismality" has evolved socially, as groups of simpler units (from cells upwards) came to cooperate without conflicts. They propose that cooperation should be used as

2328-571: Is a type of metabolism found in prokaryotes where energy is obtained from the oxidation of inorganic compounds . These organisms can use hydrogen , reduced sulfur compounds (such as sulfide , hydrogen sulfide and thiosulfate ), ferrous iron (Fe(II)) or ammonia as sources of reducing power and they gain energy from the oxidation of these compounds. These microbial processes are important in global biogeochemical cycles such as acetogenesis , nitrification and denitrification and are critical for soil fertility . The energy in sunlight

2425-535: Is an argument for viewing viruses as cellular organisms. Some researchers perceive viruses not as virions alone, which they believe are just spores of an organism, but as a virocell - an ontologically mature viral organism that has cellular structure. Such virus is a result of infection of a cell and shows all major physiological properties of other organisms: metabolism , growth, and reproduction , therefore, life in its effective presence. The philosopher Jack A. Wilson examines some boundary cases to demonstrate that

2522-401: Is an organic compound needed in small quantities that cannot be made in cells. In human nutrition , most vitamins function as coenzymes after modification; for example, all water-soluble vitamins are phosphorylated or are coupled to nucleotides when they are used in cells. Nicotinamide adenine dinucleotide (NAD ), a derivative of vitamin B 3 ( niacin ), is an important coenzyme that acts as

2619-446: Is called gluconeogenesis . Gluconeogenesis converts pyruvate to glucose-6-phosphate through a series of intermediates, many of which are shared with glycolysis . However, this pathway is not simply glycolysis run in reverse, as several steps are catalyzed by non-glycolytic enzymes. This is important as it allows the formation and breakdown of glucose to be regulated separately, and prevents both pathways from running simultaneously in

2716-409: Is captured by plants , cyanobacteria , purple bacteria , green sulfur bacteria and some protists . This process is often coupled to the conversion of carbon dioxide into organic compounds, as part of photosynthesis, which is discussed below. The energy capture and carbon fixation systems can, however, operate separately in prokaryotes, as purple bacteria and green sulfur bacteria can use sunlight as

2813-405: Is carried out by a particular coenzyme, which is the substrate for a set of enzymes that produce it, and a set of enzymes that consume it. These coenzymes are therefore continuously made, consumed and then recycled. One central coenzyme is adenosine triphosphate (ATP), the energy currency of cells. This nucleotide is used to transfer chemical energy between different chemical reactions. There

2910-638: Is carried out by the enzyme RuBisCO as part of the Calvin–Benson cycle . Three types of photosynthesis occur in plants, C3 carbon fixation , C4 carbon fixation and CAM photosynthesis . These differ by the route that carbon dioxide takes to the Calvin cycle, with C3 plants fixing CO 2 directly, while C4 and CAM photosynthesis incorporate the CO 2 into other compounds first, as adaptations to deal with intense sunlight and dry conditions. In photosynthetic prokaryotes

3007-485: Is composed of a phosphate attached to a ribose or deoxyribose sugar group which is attached to a nitrogenous base . Nucleic acids are critical for the storage and use of genetic information, and its interpretation through the processes of transcription and protein biosynthesis . This information is protected by DNA repair mechanisms and propagated through DNA replication . Many viruses have an RNA genome , such as HIV , which uses reverse transcription to create

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3104-568: Is inadequate in biology; that the concept of individuality is problematic; and from a philosophical point of view, question whether such a definition is necessary. Problematic cases include colonial organisms : for instance, a colony of eusocial insects fulfills criteria such as adaptive organisation and germ-soma specialisation. If so, the same argument, or a criterion of high co-operation and low conflict, would include some mutualistic (e.g. lichens) and sexual partnerships (e.g. anglerfish ) as organisms. If group selection occurs, then

3201-477: Is its primary structure . Just as the letters of the alphabet can be combined to form an almost endless variety of words, amino acids can be linked in varying sequences to form a huge variety of proteins. Proteins are made from amino acids that have been activated by attachment to a transfer RNA molecule through an ester bond. This aminoacyl-tRNA precursor is produced in an ATP -dependent reaction carried out by an aminoacyl tRNA synthetase . This aminoacyl-tRNA

3298-635: Is likely due to their efficacy . In various diseases, such as type II diabetes , metabolic syndrome , and cancer , normal metabolism is disrupted. The metabolism of cancer cells is also different from the metabolism of normal cells, and these differences can be used to find targets for therapeutic intervention in cancer. Most of the structures that make up animals, plants and microbes are made from four basic classes of molecules : amino acids , carbohydrates , nucleic acid and lipids (often called fats ). As these molecules are vital for life, metabolic reactions either focus on making these molecules during

3395-472: Is likely intrinsic to life. Thus, the earliest organisms also presumably possessed a self-replicating informational molecule ( genome ), perhaps RNA or an informational molecule more primitive than RNA. The specific nucleotide sequences in all currently extant organisms contain information that functions to promote survival, reproduction , and the ability to acquire resources necessary for reproduction, and sequences with such functions probably emerged early in

3492-559: Is needed, or back to glucose in the Cori cycle . An alternative route for glucose breakdown is the pentose phosphate pathway , which produces less energy but supports anabolism (biomolecule synthesis). This pathway reduces the coenzyme NADP to NADPH and produces pentose compounds such as ribose 5-phosphate for synthesis of many biomolecules such as nucleotides and aromatic amino acids . Fats are catabolized by hydrolysis to free fatty acids and glycerol. The glycerol enters glycolysis and

3589-475: Is only a small amount of ATP in cells, but as it is continuously regenerated, the human body can use about its own weight in ATP per day. ATP acts as a bridge between catabolism and anabolism . Catabolism breaks down molecules, and anabolism puts them together. Catabolic reactions generate ATP, and anabolic reactions consume it. It also serves as a carrier of phosphate groups in phosphorylation reactions. A vitamin

3686-411: Is produced in response to rises in blood glucose levels . Binding of the hormone to insulin receptors on cells then activates a cascade of protein kinases that cause the cells to take up glucose and convert it into storage molecules such as fatty acids and glycogen . The metabolism of glycogen is controlled by activity of phosphorylase , the enzyme that breaks down glycogen, and glycogen synthase ,

3783-546: Is scarce, or when cells undergo metabolic stress. Lipids are the most diverse group of biochemicals. Their main structural uses are as part of internal and external biological membranes , such as the cell membrane . Their chemical energy can also be used. Lipids contain a long, non-polar hydrocarbon chain with a small polar region containing oxygen. Lipids are usually defined as hydrophobic or amphipathic biological molecules but will dissolve in organic solvents such as ethanol , benzene or chloroform . The fats are

3880-617: Is the measure of the amount of energy consumed by all of these chemical reactions. A striking feature of metabolism is the similarity of the basic metabolic pathways among vastly different species. For example, the set of carboxylic acids that are best known as the intermediates in the citric acid cycle are present in all known organisms, being found in species as diverse as the unicellular bacterium Escherichia coli and huge multicellular organisms like elephants . These similarities in metabolic pathways are likely due to their early appearance in evolutionary history , and their retention

3977-422: Is the synthesis of carbohydrates from sunlight and carbon dioxide (CO 2 ). In plants, cyanobacteria and algae, oxygenic photosynthesis splits water, with oxygen produced as a waste product. This process uses the ATP and NADPH produced by the photosynthetic reaction centres , as described above, to convert CO 2 into glycerate 3-phosphate , which can then be converted into glucose. This carbon-fixation reaction

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4074-495: Is then a substrate for the ribosome , which joins the amino acid onto the elongating protein chain, using the sequence information in a messenger RNA . Nucleotides are made from amino acids, carbon dioxide and formic acid in pathways that require large amounts of metabolic energy. Consequently, most organisms have efficient systems to salvage preformed nucleotides. Purines are synthesized as nucleosides (bases attached to ribose ). Both adenine and guanine are made from

4171-423: Is transformed through a series of steps into another chemical, each step being facilitated by a specific enzyme . Enzymes are crucial to metabolism because they allow organisms to drive desirable reactions that require energy and will not occur by themselves, by coupling them to spontaneous reactions that release energy. Enzymes act as catalysts —they allow a reaction to proceed more rapidly—and they also allow

4268-429: Is used in anabolic reactions. Inorganic elements play critical roles in metabolism; some are abundant (e.g. sodium and potassium ) while others function at minute concentrations. About 99% of a human's body weight is made up of the elements carbon , nitrogen , calcium , sodium , chlorine , potassium , hydrogen , phosphorus , oxygen and sulfur . Organic compounds (proteins, lipids and carbohydrates) contain

4365-426: The bioremediation of contaminated land and oil spills. Many of these microbial reactions are shared with multicellular organisms, but due to the incredible diversity of types of microbes these organisms are able to deal with a far wider range of xenobiotics than multicellular organisms, and can degrade even persistent organic pollutants such as organochloride compounds. A related problem for aerobic organisms

4462-499: The chloroplast . These protons move back through the membrane as they drive the ATP synthase, as before. The electrons then flow through photosystem I and can then be used to reduce the coenzyme NADP . This coenzyme can enter the Calvin cycle or be recycled for further ATP generation. Anabolism is the set of constructive metabolic processes where the energy released by catabolism is used to synthesize complex molecules. In general,

4559-467: The control exerted by this enzyme is the effect that these changes in its activity have on the overall rate of the pathway (the flux through the pathway). For example, an enzyme may show large changes in activity (i.e. it is highly regulated) but if these changes have little effect on the flux of a metabolic pathway, then this enzyme is not involved in the control of the pathway. There are multiple levels of metabolic regulation. In intrinsic regulation,

4656-441: The cytoskeleton , a system of scaffolding that maintains the cell shape. Proteins are also important in cell signaling , immune responses , cell adhesion , active transport across membranes, and the cell cycle . Amino acids also contribute to cellular energy metabolism by providing a carbon source for entry into the citric acid cycle ( tricarboxylic acid cycle ), especially when a primary source of energy, such as glucose ,

4753-406: The last universal common ancestor . This universal ancestral cell was prokaryotic and probably a methanogen that had extensive amino acid, nucleotide, carbohydrate and lipid metabolism. The retention of these ancient pathways during later evolution may be the result of these reactions having been an optimal solution to their particular metabolic problems, with pathways such as glycolysis and

4850-550: The mevalonate pathway produces these compounds from acetyl-CoA, while in plants and bacteria the non-mevalonate pathway uses pyruvate and glyceraldehyde 3-phosphate as substrates. One important reaction that uses these activated isoprene donors is sterol biosynthesis . Here, the isoprene units are joined to make squalene and then folded up and formed into a set of rings to make lanosterol . Lanosterol can then be converted into other sterols such as cholesterol and ergosterol . Organisms vary in their ability to synthesize

4947-426: The regulation of the rate of a metabolic reaction, for example in response to changes in the cell's environment or to signals from other cells. The metabolic system of a particular organism determines which substances it will find nutritious and which poisonous . For example, some prokaryotes use hydrogen sulfide as a nutrient, yet this gas is poisonous to animals. The basal metabolic rate of an organism

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5044-407: The stomach and pancreas , and in salivary glands . The amino acids or sugars released by these extracellular enzymes are then pumped into cells by active transport proteins. Carbohydrate catabolism is the breakdown of carbohydrates into smaller units. Carbohydrates are usually taken into cells after they have been digested into monosaccharides such as glucose and fructose . Once inside,

5141-642: The "defining trait" of an organism. Samuel Díaz‐Muñoz and colleagues (2016) accept Queller and Strassmann's view that organismality can be measured wholly by degrees of cooperation and of conflict. They state that this situates organisms in evolutionary time, so that organismality is context dependent. They suggest that highly integrated life forms, which are not context dependent, may evolve through context-dependent stages towards complete unification. Viruses are not typically considered to be organisms, because they are incapable of autonomous reproduction , growth , metabolism , or homeostasis . Although viruses have

5238-466: The "defining trait" of an organism. This would treat many types of collaboration, including the fungus / alga partnership of different species in a lichen , or the permanent sexual partnership of an anglerfish , as an organism. The term "organism" (from the Ancient Greek ὀργανισμός , derived from órganon , meaning instrument, implement, tool, organ of sense or apprehension) first appeared in

5335-428: The 20 common amino acids. Most bacteria and plants can synthesize all twenty, but mammals can only synthesize eleven nonessential amino acids, so nine essential amino acids must be obtained from food. Some simple parasites , such as the bacteria Mycoplasma pneumoniae , lack all amino acid synthesis and take their amino acids directly from their hosts. All amino acids are synthesized from intermediates in glycolysis,

5432-452: The English language in the 1660s with the now-obsolete meaning of an organic structure or organization. It is related to the verb "organize". In his 1790 Critique of Judgment , Immanuel Kant defined an organism as "both an organized and a self-organizing being". Among the criteria that have been proposed for being an organism are: Other scientists think that the concept of the organism

5529-848: The Institute for Biomedical Problems of the Russian Academy of Sciences , the Moscow State University , the American Type Culture Collection (ATCC), and the Institute for Aerospace Medicine in Germany. Three fundamental guidelines governed the selection of the organisms: First, the organisms selected represent the three domains of life – eukaryote , bacteria and archaea . Second, the organisms are very well studied (e.g., having their genome sequenced and studied in many other experiments) to make it possible to accurately assess

5626-431: The brain that cannot metabolize fatty acids. In other organisms such as plants and bacteria, this metabolic problem is solved using the glyoxylate cycle , which bypasses the decarboxylation step in the citric acid cycle and allows the transformation of acetyl-CoA to oxaloacetate , where it can be used for the production of glucose. Other than fat, glucose is stored in most tissues, as an energy resource available within

5723-439: The cell for energy. M. tuberculosis can also grow on the lipid cholesterol as a sole source of carbon, and genes involved in the cholesterol-use pathway(s) have been validated as important during various stages of the infection lifecycle of M. tuberculosis . Amino acids are either used to synthesize proteins and other biomolecules, or oxidized to urea and carbon dioxide to produce energy. The oxidation pathway starts with

5820-441: The citric acid cycle producing their end products highly efficiently and in a minimal number of steps. The first pathways of enzyme-based metabolism may have been parts of purine nucleotide metabolism, while previous metabolic pathways were a part of the ancient RNA world . Many models have been proposed to describe the mechanisms by which novel metabolic pathways evolve. These include the sequential addition of novel enzymes to

5917-416: The citric acid cycle, or the pentose phosphate pathway. Nitrogen is provided by glutamate and glutamine . Nonessensial amino acid synthesis depends on the formation of the appropriate alpha-keto acid, which is then transaminated to form an amino acid. Amino acids are made into proteins by being joined in a chain of peptide bonds . Each different protein has a unique sequence of amino acid residues: this

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6014-637: The coenzyme nicotinamide adenine dinucleotide (NAD ) into NADH. Macromolecules cannot be directly processed by cells. Macromolecules must be broken into smaller units before they can be used in cell metabolism. Different classes of enzymes are used to digest these polymers. These digestive enzymes include proteases that digest proteins into amino acids, as well as glycoside hydrolases that digest polysaccharides into simple sugars known as monosaccharides . Microbes simply secrete digestive enzymes into their surroundings, while animals only secrete these enzymes from specialized cells in their guts , including

6111-534: The complex molecules that make up cellular structures are constructed step-by-step from smaller and simpler precursors. Anabolism involves three basic stages. First, the production of precursors such as amino acids , monosaccharides , isoprenoids and nucleotides , secondly, their activation into reactive forms using energy from ATP, and thirdly, the assembly of these precursors into complex molecules such as proteins , polysaccharides , lipids and nucleic acids . Anabolism in organisms can be different according to

6208-678: The concept of organism is not sharply defined. In his view, sponges , lichens , siphonophores , slime moulds , and eusocial colonies such as those of ants or naked molerats , all lie in the boundary zone between being definite colonies and definite organisms (or superorganisms). Scientists and bio-engineers are experimenting with different types of synthetic organism , from chimaeras composed of cells from two or more species, cyborgs including electromechanical limbs, hybrots containing both electronic and biological elements, and other combinations of systems that have variously evolved and been designed. An evolved organism takes its form by

6305-488: The construction of cells and tissues, or on breaking them down and using them to obtain energy, by their digestion. These biochemicals can be joined to make polymers such as DNA and proteins , essential macromolecules of life. Proteins are made of amino acids arranged in a linear chain joined by peptide bonds . Many proteins are enzymes that catalyze the chemical reactions in metabolism. Other proteins have structural or mechanical functions, such as those that form

6402-422: The conversion of food to building blocks of proteins , lipids , nucleic acids , and some carbohydrates ; and the elimination of metabolic wastes . These enzyme -catalyzed reactions allow organisms to grow and reproduce, maintain their structures , and respond to their environments. The word metabolism can also refer to the sum of all chemical reactions that occur in living organisms, including digestion and

6499-455: The effects of the long exposure to space. If they had already been studied in space conditions so much the better, since it would enable researchers to pinpoint precisely how organisms were affected by the years-long exposure to the interplanetary environment. Finally, a strong preference was given to organisms that appear to stand the best chance of surviving the journey. These are extremophiles , organisms that thrive in conditions that would kill

6596-445: The electrons removed from organic molecules in areas such as the citric acid cycle are transferred to oxygen and the energy released is used to make ATP. This is done in eukaryotes by a series of proteins in the membranes of mitochondria called the electron transport chain . In prokaryotes , these proteins are found in the cell's inner membrane . These proteins use the energy from reduced molecules like NADH to pump protons across

6693-683: The end of the reaction catalyzed. Metal micronutrients are taken up into organisms by specific transporters and bind to storage proteins such as ferritin or metallothionein when not in use. Catabolism is the set of metabolic processes that break down large molecules. These include breaking down and oxidizing food molecules. The purpose of the catabolic reactions is to provide the energy and components needed by anabolic reactions which build molecules. The exact nature of these catabolic reactions differ from organism to organism, and organisms can be classified based on their sources of energy, hydrogen, and carbon (their primary nutritional groups ), as shown in

6790-469: The enzyme that makes it. These enzymes are regulated in a reciprocal fashion, with phosphorylation inhibiting glycogen synthase, but activating phosphorylase. Insulin causes glycogen synthesis by activating protein phosphatases and producing a decrease in the phosphorylation of these enzymes. The central pathways of metabolism described above, such as glycolysis and the citric acid cycle, are present in all three domains of living things and were present in

6887-422: The evolution of life. It is also likely that survival sequences present early in the evolution of organisms included sequences that facilitate the avoidance of damage to the self-replicating molecule and promote the capability to repair such damages that do occur. Repair of some of the genome damages in these early organisms may have involved the capacity to use undamaged information from another similar genome by

6984-650: The exchange of electrolytes between the extracellular fluid and the cell's fluid, the cytosol . Electrolytes enter and leave cells through proteins in the cell membrane called ion channels . For example, muscle contraction depends upon the movement of calcium, sodium and potassium through ion channels in the cell membrane and T-tubules . Transition metals are usually present as trace elements in organisms, with zinc and iron being most abundant of those. Metal cofactors are bound tightly to specific sites in proteins; although enzyme cofactors can be modified during catalysis, they always return to their original state by

7081-414: The fatty acids are broken down by beta oxidation to release acetyl-CoA, which then is fed into the citric acid cycle. Fatty acids release more energy upon oxidation than carbohydrates. Steroids are also broken down by some bacteria in a process similar to beta oxidation, and this breakdown process involves the release of significant amounts of acetyl-CoA, propionyl-CoA, and pyruvate, which can all be used by

7178-484: The first stage, large organic molecules, such as proteins , polysaccharides or lipids , are digested into their smaller components outside cells. Next, these smaller molecules are taken up by cells and converted to smaller molecules, usually acetyl coenzyme A (acetyl-CoA), which releases some energy. Finally, the acetyl group on acetyl-CoA is oxidized to water and carbon dioxide in the citric acid cycle and electron transport chain , releasing more energy while reducing

7275-405: The form of water-soluble messengers such as hormones and growth factors and are detected by specific receptors on the cell surface. These signals are then transmitted inside the cell by second messenger systems that often involved the phosphorylation of proteins. A very well understood example of extrinsic control is the regulation of glucose metabolism by the hormone insulin . Insulin

7372-529: The major route of breakdown is glycolysis , in which glucose is converted into pyruvate . This process generates the energy-conveying molecule NADH from NAD , and generates ATP from ADP for use in powering many processes within the cell. Pyruvate is an intermediate in several metabolic pathways, but the majority is converted to acetyl-CoA and fed into the citric acid cycle , which enables more ATP production by means of oxidative phosphorylation . This oxidation consumes molecular oxygen and releases water and

7469-506: The majority of the carbon and nitrogen; most of the oxygen and hydrogen is present as water. The abundant inorganic elements act as electrolytes . The most important ions are sodium , potassium , calcium , magnesium , chloride , phosphate and the organic ion bicarbonate . The maintenance of precise ion gradients across cell membranes maintains osmotic pressure and pH . Ions are also critical for nerve and muscle function, as action potentials in these tissues are produced by

7566-680: The mechanisms of carbon fixation are more diverse. Here, carbon dioxide can be fixed by the Calvin–Benson cycle, a reversed citric acid cycle, or the carboxylation of acetyl-CoA. Prokaryotic chemoautotrophs also fix CO 2 through the Calvin–Benson cycle, but use energy from inorganic compounds to drive the reaction. In carbohydrate anabolism, simple organic acids can be converted into monosaccharides such as glucose and then used to assemble polysaccharides such as starch . The generation of glucose from compounds like pyruvate , lactate , glycerol , glycerate 3-phosphate and amino acids

7663-482: The metabolic pathway self-regulates to respond to changes in the levels of substrates or products; for example, a decrease in the amount of product can increase the flux through the pathway to compensate. This type of regulation often involves allosteric regulation of the activities of multiple enzymes in the pathway. Extrinsic control involves a cell in a multicellular organism changing its metabolism in response to signals from other cells. These signals are usually in

7760-512: The most abundant biological molecules, and fill numerous roles, such as the storage and transport of energy ( starch , glycogen ) and structural components ( cellulose in plants, chitin in animals). The basic carbohydrate units are called monosaccharides and include galactose , fructose , and most importantly glucose . Monosaccharides can be linked together to form polysaccharides in almost limitless ways. The two nucleic acids, DNA and RNA , are polymers of nucleotides . Each nucleotide

7857-409: The most common is that an organism has autonomous reproduction , growth , and metabolism . This would exclude viruses , despite the fact that they evolve like organisms. Other problematic cases include colonial organisms ; a colony of eusocial insects is organised adaptively, and has germ-soma specialisation , with some insects reproducing, others not, like cells in an animal's body. The body of

7954-420: The nature of photosynthetic pigment present, with most photosynthetic bacteria only having one type, while plants and cyanobacteria have two. In plants, algae, and cyanobacteria, photosystem II uses light energy to remove electrons from water, releasing oxygen as a waste product. The electrons then flow to the cytochrome b6f complex , which uses their energy to pump protons across the thylakoid membrane in

8051-501: The partially understood mechanisms of evolutionary developmental biology , in which the genome directs an elaborated series of interactions to produce successively more elaborate structures. The existence of chimaeras and hybrids demonstrates that these mechanisms are "intelligently" robust in the face of radically altered circumstances at all levels from molecular to organismal. Synthetic organisms already take diverse forms, and their diversity will increase. What they all have in common

8148-412: The pathway. Terpenes and isoprenoids are a large class of lipids that include the carotenoids and form the largest class of plant natural products . These compounds are made by the assembly and modification of isoprene units donated from the reactive precursors isopentenyl pyrophosphate and dimethylallyl pyrophosphate . These precursors can be made in different ways. In animals and archaea,

8245-674: The precursor nucleoside inosine monophosphate, which is synthesized using atoms from the amino acids glycine , glutamine , and aspartic acid , as well as formate transferred from the coenzyme tetrahydrofolate . Pyrimidines , on the other hand, are synthesized from the base orotate , which is formed from glutamine and aspartate. All organisms are constantly exposed to compounds that they cannot use as foods and that would be harmful if they accumulated in cells, as they have no metabolic function. These potentially damaging compounds are called xenobiotics . Xenobiotics such as synthetic drugs , natural poisons and antibiotics are detoxified by

8342-430: The removal of the amino group by a transaminase . The amino group is fed into the urea cycle , leaving a deaminated carbon skeleton in the form of a keto acid . Several of these keto acids are intermediates in the citric acid cycle, for example α- ketoglutarate formed by deamination of glutamate . The glucogenic amino acids can also be converted into glucose, through gluconeogenesis . In oxidative phosphorylation,

8439-627: The source of constructed molecules in their cells. Autotrophs such as plants can construct the complex organic molecules in their cells such as polysaccharides and proteins from simple molecules like carbon dioxide and water. Heterotrophs , on the other hand, require a source of more complex substances, such as monosaccharides and amino acids, to produce these complex molecules. Organisms can be further classified by ultimate source of their energy: photoautotrophs and photoheterotrophs obtain energy from light, whereas chemoautotrophs and chemoheterotrophs obtain energy from oxidation reactions. Photosynthesis

8536-425: The substrate can be acceptors, the polysaccharides produced can have straight or branched structures. The polysaccharides produced can have structural or metabolic functions themselves, or be transferred to lipids and proteins by the enzymes oligosaccharyltransferases . Fatty acids are made by fatty acid synthases that polymerize and then reduce acetyl-CoA units. The acyl chains in the fatty acids are extended by

8633-826: The table below. Organic molecules are used as a source of hydrogen atoms or electrons by organotrophs , while lithotrophs use inorganic substrates. Whereas phototrophs convert sunlight to chemical energy , chemotrophs depend on redox reactions that involve the transfer of electrons from reduced donor molecules such as organic molecules , hydrogen , hydrogen sulfide or ferrous ions to oxygen , nitrate or sulfate . In animals, these reactions involve complex organic molecules that are broken down to simpler molecules, such as carbon dioxide and water. Photosynthetic organisms, such as plants and cyanobacteria , use similar electron-transfer reactions to store energy absorbed from sunlight. The most common set of catabolic reactions in animals can be separated into three main stages. In

8730-407: The tissue through glycogenesis which was usually being used to maintained glucose level in blood. Polysaccharides and glycans are made by the sequential addition of monosaccharides by glycosyltransferase from a reactive sugar-phosphate donor such as uridine diphosphate glucose (UDP-Glc) to an acceptor hydroxyl group on the growing polysaccharide. As any of the hydroxyl groups on the ring of

8827-480: The transfer of heat and work . The second law of thermodynamics states that in any isolated system , the amount of entropy (disorder) cannot decrease. Although living organisms' amazing complexity appears to contradict this law, life is possible as all organisms are open systems that exchange matter and energy with their surroundings. Living systems are not in equilibrium , but instead are dissipative systems that maintain their state of high complexity by causing

8924-637: The transportation of substances into and between different cells, in which case the above described set of reactions within the cells is called intermediary (or intermediate) metabolism. Metabolic reactions may be categorized as catabolic —the breaking down of compounds (for example, of glucose to pyruvate by cellular respiration ); or anabolic —the building up ( synthesis ) of compounds (such as proteins, carbohydrates, lipids, and nucleic acids). Usually, catabolism releases energy, and anabolism consumes energy. The chemical reactions of metabolism are organized into metabolic pathways , in which one chemical

9021-534: The vast majority of Earthly creatures. The 10 'passenger' organisms selected are listed below: Bacteria Archaea Eukaryote The mass of the Bio-Module on board the Fobos-Grunt spacecraft was 100 grams or less. The design is a short cylinder . The bio-module provided 30 small tubes (3 millimeters in diameter) for individual microbe samples. It also accommodated a native sample of bacteria – derived from

9118-465: The waste product carbon dioxide. When oxygen is lacking, or when pyruvate is temporarily produced faster than it can be consumed by the citric acid cycle (as in intense muscular exertion), pyruvate is converted to lactate by the enzyme lactate dehydrogenase , a process that also oxidizes NADH back to NAD for re-use in further glycolysis, allowing energy production to continue. The lactate is later converted back to pyruvate for ATP production where energy

9215-459: Was co-evolution of viruses and host cells. If host cells did not exist, viral evolution would be impossible. As for reproduction, viruses rely on hosts' machinery to replicate. The discovery of viruses with genes coding for energy metabolism and protein synthesis fuelled the debate about whether viruses are living organisms, but the genes have a cellular origin. Most likely, they were acquired through horizontal gene transfer from viral hosts. There

9312-561: Was successfully flown in 2011 aboard the final flight of Space Shuttle Endeavour , STS-134 . Known as the Shuttle-LIFE (also LIFE ) experiment. The project includes representatives of all three domains of life : bacteria , eukaryota and archaea . The capsule was transporting 10 types of organisms in 30 self-contained samples, i.e., each in triplicate. In addition, one or more natural native soil samples were flown in their own self-contained capsule. The Phobos-Soil sample return mission

9409-489: Was the only attempted biological science mission that would have returned to Earth from deep space, far beyond the protection of Earth's magnetic field; sending biological samples through deep space is therefore a much better test of interplanetary survivability than sending the samples on a typical Earth-orbiting flight. The project was being done in collaboration with the Russian Space Research Institute ,

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